Etching is a technique used in patterning and other applications in a wide range of fields such as printing, semiconductor devices, and printed wire boards, and is indispensable as an underlying technique for microfabrication. Etching of metal can be broadly classified into dry etching that uses a plasma gas, and wet etching that uses chemical reaction in liquid.
Dry etching is suited for microfabrication but requires a large capital investment, and it is difficult to apply this technique to large area processes because of the limitations in process area. Further, dry etching of metal is an inherently difficult process, and this has led to the rise of the damascene process, which does not require dry etching metal, in the ultrafine wire fabrication of recent semiconductor devices.
On the other hand, wet etching is unsuited for miniaturization because it involves slower diffusion into a microstructure than in gases, and structural deformation and destruction that occur because of surface tension or other properties during a drying process. Another problem is the large environmental burden due to waste fluid. However, despite these problems, wet etching has been commonly used for its advantage of allowing etching over a large area, and represents a technique indispensable for the production of the wiring patterns and the printing plate of circuit boards in printed electronics.
A washing technique using more environmentally friendly ozone oxidation has attracted interest in a washing process that removes organic materials. Ozone self-decomposes into oxygen in the atmosphere, and has a small environmental burden. This, together with the strong oxidative power, has recently made ozone useful in applications such as removal of organic materials, and sterilization in sewage processes and medical sterilization using its sterilizing effect (see Non Patent Literature 1 and 2). In semiconductor industry, ozone has attracted interest as a material for green technology in washing display panels and removing photoresist (see Non Patent Literature 3 and 4).
Various techniques are known for the washing of display panels and the removal of photoresist using ozone water. Patent Literature 1 discloses performing an ozone water pretreatment and removing fine resist residues adhering to surfaces of a target etching material (metallic material, metal oxide) having resist patterns before etching. Patent Literature 2 discloses supplying high concentration ozone water to a substrate to remove organic materials, metal contaminants, and other such materials adhering to a substrate surface. It is also known to ionize and remove scraped copper with ozone water when smoothing a copper surface with a polishing pad on a logic circuit formed with copper wires (see Patent Literature 3).
However, in the invention described in Patent Literature 1, ozone water is used to remove resist material. Patent Literature 2 describes removing residues such as metal contaminants with ozone water. Patent Literature 3 describes ionizing and removing copper with ozone water. The removal of metal with ozone water as currently practiced in the art uses ozone water not by itself but in combination with hydrochloric acid or hydrofluoric acid (Patent Literature 4, Non Patent Literature 5). In actual practice, the inventions described in Patent Literature 2 and 3 also require addition of hydrochloric acid or hydrofluoric acid, and the environmental issues have not been completely solved.
With the recent advancement in nanotechnology, techniques of manipulating metal at an atomic level have been more important, such as in forming wires through etching of metal at an atomic level, and smoothing a metal surface at an atomic level. However, no method is available that can achieve these goals without using environmentally harmful reagents or materials.